This invention relates to a heat pump system for use in the heating and/or cooling of homes, apartments and other buildings, such as industrial plants and complexes (whether public or private) in which people live or work. More specifically, the invention is concerned with buildings that require both heating or cooling of the air space within the building, as well as the further heating of a body of water, principally the provision of a hot water supply for normal bathroom, kitchen and utility room use. In addition some systems may provide for the heating of additional water, such as that of a swimming pool, hot tube etc.